1. Field of the Invention
The present invention relates to light guides for illumination, and more particularly to a tapered collimating light guide for flat panel displays.
2. Description of the Related Art
In a typical portable display backlight unit, a triband cold cathode fluorescent lamp (CCFL) is coupled to one edge of a tapered Acrylic light guide where a white reflector and a variable density pattern of printed dots or molded microstructure such as ridges, etc. are formed on the bottom surface of the light guide to scatter light and generate a uniform distribution of light which exits the light guide.
Referring to FIG. 1, a side view of one display arrangement is shown. A light source 10 is coupled to a tapered light guide 12 for introducing light into light guide 12. Light source 10 includes a cold cathode flourescent lamp (CCFL) which is elongated into and out of the plane of the page. Source 10 also typically includes a reflector 14 for directing light into light guide 12. A dot pattern or microstructures 16 are formed on a surface 18 to diffuse light incident on surface 18. Light 11 exits guide 12 in twin peaks at angles of about 50 degrees and 80 degrees off normal with a full width half maximum distribution (FWHM) of xc2x130 degrees in the case of a printed white xe2x80x9cdotxe2x80x9d pattern on surface 18 and in twin peaks at angles of about 60 degrees and about 80 degrees off normal with a FWHM distribution of xc2x120 degrees in the case of microstructured ridges on surface 18. A white reflector sheet 20 is spaced apart from and runs parallel to surface 18 to redirect light back towards guide 12. Typically, a pair of crossed ridge sheets 22 and 24 with a diffuser sheet 23 are used to redirect the light normal to light guide 12 and to somewhat collimate the light (about xc2x125 degrees FWHM horizontal and vertical). Ridge sheets 22 and 24 include parallel running spaced ridges. Ridges on sheet 22 run perpendicular to ridges on sheet 24.
Recently, an alternate approach using a prism bending sheet has been disclosed by Mitsubishi Rayon in U.S. Pat. Nos. Re 35,704, 5,711,589, and 5,863,113. In these patents, the pair of crossed xe2x80x9cridge sheetsxe2x80x9d is replaced by a single prism bending sheet 30, as shown in FIG. 2.
Referring to FIG. 2, prism bending sheet 30 redirects the light which exits the light guide into a direction normal to the light guide where surface texture or microstructure variation along the light guide on surface 34 is used to produce a uniform light distribution. The use of a bending sheet 30 has significant advantages in that only one sheet is required instead of two, and it can result in more collimated light in the direction along the length of the guide, but the degree of collimation which can be achieved is limited by the surface texture or microstructure along the guide which controls the extraction of light to achieve uniform illumination. The prism bending sheet preserves the degree of collimation of the light output by the light guide in the total internal reflection (TIR) direction unlike the ridge sheets as shown in FIG. 1.
A very high degree of collimation in the direction along the guide can be achieved by a tapered light guide with both major surfaces flat and with a mirror underneath instead of a white reflector, but the light distribution along the length of the guide is very nonuniform.
Conventional backlight designs provide a flat entrance surface 13 angled at about 90 degrees with the top surface (exit surface 15) of the light guides 2 of FIGS. 1 and 2. With reference to the coordinate system of FIG. 2, FIGS. 3A and 3B shown the interface between air and light guide 12 at entrance edge 13. Light enters at entrance edge 13 for xe2x88x9290 degrees xe2x89xa6xcex1xe2x89xa690 degrees and xe2x88x9290 degrees xe2x89xa6xcex2xe2x89xa690 degrees, where xcex1 is the input angle of light from the x-axis in the x-y plane and xcex2 in the input angle of light from the x-axis in the x-z plane. xcex1xe2x80x2 and xcex2xe2x80x2 are corresponding angles in the light guide after refraction. The angular distribution of light is reduced according to Snell""s law when light propagates into light guide 12, which has a higher index of refraction than air (e.g., n=1.49 for acrylic). If entrance edge 13 is 3.5 mm thick, taper angle is 0.5 degrees surface 16 and n=1.49 for light guide 12, then the initial light distribution in the acrylic light guide is xe2x88x9242 degrees xe2x89xa6xcex1xe2x80x2xe2x89xa642 degrees and xe2x88x9242 degrees xe2x89xa6xcex2xe2x80x2xe2x89xa642. Considering only the x-y plane, xcex1xe2x80x2 is increased each time propagating light is reflected from surface 16 until the angle for TIR (e.g. 48 degrees) is exceeded. If structures of the prior art did not include a dot pattern or microstructure on surface 16 (See FIGS. 1 and 2), light must propagate about 40 mm in the x direction before TIR is exceeded. This creates an undesirable dark region adjacent to entrance edge 13.
There are a number of applications where it is desirable to have very highly collimated light exit the light guide. The primary application is to make a xe2x80x9ccollimate and post diffusexe2x80x9d (CPD) type of display as has been described previously in Zimmerman et al. in SID ""95 Digest, pp. 793-796 and McFarland et al. in Asia Display ""95 Digest, pp. 739-742. The system as described in these publications and their respective patent (e.g., U.S. Pat. No. 5,598,281) is not practical for many applications due to the thickness and cost required to manufacture the complicated structures described therein.
A number of recent publications including Kalantar et al. SID ""99 Digest, pp. 764, Kalantar et al. SID ""00 Digest pp.1029-1031, have described that by forming ridges along one major surface of the light guide perpendicular to the axis of the CCFL (axis running into the page for example in FIGS. 1 and 2), the output light can be collimated in the direction across the guide (where xe2x80x9cacross the guidexe2x80x9d is perpendicular to xe2x80x9calong the guidexe2x80x9d which is perpendicular to the axis of the CCFL). See also U.S. Pat. No. 5,926,601.
Therefore, a need exists for a light guide system which efficiently provides a more uniform light distribution at the output of a light guide while maintaining a highly collimated light output.
A light guide system has a light guide having a first end portion opposite a second end portion. The light guide provides a first surface and a second surface between the first and second end portions, and the second surface is inclined relative to the first surface. A light source is disposed along the first end portion on a first axis. A light redistribution device is disposed at an entrance of the light guide for receiving light from the light source and redistributing a portion of the light perpendicular to the first axis to provide a uniform light distribution from the first surface.
Another light guide system includes a light guide having a first end portion opposite a second end portion, and the light guide includes a first surface and a second surface between the first and second end portions. The second surface is inclined relative to the first surface. A light source is disposed along the first end portion wherein the first end portion defines a first axis and the light source extends along the first axis. A structured entrance is formed in the first end portion of the light guide, and the structured entrance includes a plurality of angled surfaces extending along the first axis for redirecting light received from the light source.
A light guide system for liquid crystal displays includes a light guide having a first end portion opposite a second end portion, the light guide including a first surface and a second surface between the first and second end portions, the second surface being inclined relative to the first surface. A light source is disposed along the first end portion wherein the first end portion defines a first axis and the light source extends along the first axis. A reflector surrounds a portion of the light source and extends along the first axis to reflect light from the light source into the light guide. A mirror is disposed parallel to the second surface. A structured entrance is formed in the first end portion of the light guide, the structured entrance including a plurality of angled surfaces extending along the first axis for redirecting light received from the light source. A liquid crystal pixel array is disposed in operative relationship with the first surface for receiving light directed from the first surface to render an image.
In other embodiments, the light guide system includes a mirror disposed parallel to the second surface. The first surface and the second surface are preferably smooth (e.g., without roughness or dot patterns) and more preferably uniformly smooth. The second surface may be inclined relative to the first surface to produce collimated light by total internal reflection along the light guide. The light redistribution device may include a structured entrance surface for the light guide. The structured entrance surface may include a plurality of ridges extending parallel to the first axis. The plurality of ridges is preferably integrally formed in the light guide.
The structured entrance surface may include at least one facet forming an angle other than a perpendicular angle with the first surface and extending parallel to the first axis. The structured entrance surface may include at least one curved surface extending parallel to the first axis. The light guide system may include a plurality of ridges extending perpendicular to the first axis. The light guide system may include a plurality of ridges disposed on the first surface and extending perpendicular to the first axis. The light guide system may further include a black material separated by a gap from at least one of the first surface and the second surface of the light guide for absorbing a portion of the light from the light source. The plurality of ridges may include triangular shaped cross-sections. The light guide system may include a faceted reflector adapted to redirect light entering the light guide. The light guide system includes a liquid crystal display which may be a collimate and post diffuse display or a color filterless display.
These and other objects, features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings.